1. Field of the Invention
The present invention relates to improvements in a mechanical element having a shaft pressure-fitted into at least one engaging member made of sintered alloy and manufactured separately from the shaft, and more particularly, to a camshaft for internal combustion engines with relatively high torque loads, a mechanical element with its shaft hole located eccentric to the center of its engaging member(s), such as a crankshaft for compressors, and a mechanical element with its shaft hole located concentric with its engaging member(s).
The present invention also relates to a method of manufacturing the mechanical element described above.
2. Description of the Prior Art
A method for assembling a mechanical element where an engaging member(s), such as a cam made of sintered alloy, is diffusion-bonded onto a steel shaft was difficult to reduce manufacturing cost due to the requirement for a relatively large-scale furnace. Then, a method for manufacturing a camshaft by assembling an engaging member(s) and a shaft together with cutting fit which does not require a furnace, was proposed, as disclosed in U.S. Pat. Nos. 4,903,543 and 4,947,547. In this method, the camshaft is manufactured through the steps of forming a swollen portion(s) radially outwardly around a shaft with compression working method such as rolling, and bringing a cam(s) and the shaft together by pressure-fitting the cam onto the shaft so that inwardly projecting portions, that are disposed on the inside surface of a shaft hole of the cam, cut the swollen portion of the shaft to form grooves thereon.
The prior art manufacturing method, disclosed above, has several disadvantages as follows:
(1) To enable the swollen part of the shaft to be cut by the engaging member such as a cam, the shaft is required to be of mild hardness material relative to the cam, which in turn has to be of hard material, taking the necessity for wear and abrasion resistance of the cam into consideration. When a hollow shaft is employed, however, the hollow shaft tends to have to be made having a relatively greater thickness since cutting a hollow shaft of mild hardness causes decrease of the strength thereof, thereby the extent of lightening the shaft is limited.
(2) In the case where the engaging member, such as a cam, is liquid phase sintered material, known to be wear and abrasion resistant, the cam shrinks by 5 to 7% due to sintering before assembly with the shaft, thereby requiring the shaft hole to be worked after the sintering, for the purpose of maintaining accuracy of the inside diameter of the cam, which turned out to require considerable man-hours.
(3) The chip caused by the engaging member, such as the cam, cutting the swollen portion of the shaft, also required considerable man-hours to handle.
(4) In the case where the engaging member only comprises sintered alloy material, if the engaging member is not a high strength material, it often gets cracked when being assembled by cutting-fit, and, even if not cracked during the assembling, it often gets cracked when being loaded with a high torque while in use. In the case of an engaging member of high strength sintered alloy being employed to prevent such cracks, however, material and working costs tend to be extremely high. The present invention is made to solve such problems.